The invention relates to methods and devices for rapidly determining the sharpness of an image projected onto the retina of the eye. Since people who have cataracts or opacifications in their lenses or corneas complain of blurred vision even with corrective spectacles, the invention will be useful in assessing the extent of retinal image degradation experienced by these individuals and in serving as a guide for surgical intervention. As opposed to subjective vision tests, the objective tests performed with the invention do not require a verbal response from the subjects being measured.
1. Description of the Prior art
Every eye clinician qualitatively follows the progress of a patient's cataract by noting during routine ophthalmoscopic examinations how the details of fine line-like retinal blood vessels progressively blur. While these observations are useful in qualitatively assessing the extent of cataract development, they cannot easily be made quantitative since there is great variability among individuals in the size, shape and configuration of retinal blood vessels. Furthermore, simple visualization does not allow the clinician to quantify accurately the extent of blurring of the vessels.
The invention overcomes these difficulties by: 1) projecting the image of an external target source onto the patient's retina, 2) forming a second image of the retinal image of the target source onto the recording plane of a video camera, 3) transferring the electronic image to an image processing computer, and 4) measuring the extent of blurring of the retinal image of the target source with the computer. By these means, the invention provides an objective, accurate and rapid measurement of retinal image degradation due to cataract.
There are a number of optical instruments used in ophthalmic practice (including the ophthalmoscope and the slit lamp) that can be improved by the methods of the present invention in order to measure the extent of retinal image degradation due to cataract. These devices can be used to focus a test pattern on the retina, and permit the examining physician to observe the retinal image of the test pattern. Instruments of this type by Hardy, Rodenstock, Thorner, and Arnulf are described in the book by Y. Le Grand and S. G. El Hage entitled Physiological Optics, pp. 306-308, Springer-Verlag, 1980 (Berlin, Heidelberg & New York).
In Hardy's device, a target consisting of illuminated fine lines is located near the focal plane of a positive lens through which the subject views the target. The distance of the target from the lens is adjusted until the target is at maximum sharpness. This distance can be related to the spherical power correction needed by the subject. The fine line target can be rotated to check for astigmatism. In this device, the examiner looks directly into the subject's pupil without the aid of additional optical components.
Rodenstock's device is an improvement of Hardy's device. With it, the target remains fixed but its optical distance from the positive lens (through which the subject views the target) is varied through the use of a moveable reflecting prism. This device provides the examiner with a view of the image of the target on the subject's retina through the use of an afocal telescope. A focusing adjustment on the afocal telescope is linked to the prism's movement so that the image of the target on the subject's retina is simultaneously in best focus for both the subject and the examiner.
Thorner's device solves the problem of the bothersome corneal reflection of illumination light back to the examiner's eye which can interfere with the examiner's view of the retinal image of the target. The corneal reflection is eliminated by having the illumination light which forms the retinal image pass into the upper half semi-circle of the subject's pupil, and by having the returning light from the retinal image emerge from the lower half semi-circle of the subject's pupil. The two semi-circular sections of the pupil are properly apertured through the use of a pair of opposed semi-circular diaphragms in the instrument each of which is located conjugate to the appropriate half of the subject's pupil. One diaphragm is in the illumination section of the instrument while the other one is in the viewing section.
Arnulf's device uses a point source of light instead of a fine line source. Much less light enters the subject's eye with this small source and the problem of the corneal reflection is much reduced. Also, if astigmatism is present, there will be two focal positions of the instrument where sharp line-like retinal images of the point source will appear. Astigmatism can be determined by noting the focal positions of the instrument's focusing knob when sharp images of the two orthogonal line-like images are obtained. Point sources of light also have been used by Artal et al. to study such optical imaging defects in the normal eye as spherical and chromatic aberration ("Determination of the point-spread function of human eyes using a hybrid optical-digital method". Journal of the Optical Society of America A, vol.4, pp. 1109-1114 (1987); and "Optical Digital Procedure for the determination of white-light retinal images of a point test", Optical Engineering, vol. 28, pp. 687-690 (1989)).
The blurring of a line-like target on the retina of the human eye also has been measured by Campbell et al. and reported in "Optical and Retinal Factors Affecting Visual Resolution", Journal of Physiology, vol. 181, pp. 576-593 (1965), and "Optical Quality of the Human Eye", Journal of Physiology, vol. 186, pp. 558-578 (1966). In their measurements, a bright, fine line object was imaged on the retina. The light from the line image was reflected by the subject's retina back through his/her pupil and was re-imaged by an exterior lens onto a narrow, scanning slit positioned in front of a sensitive photomultiplier tube. As the slit scanned the retinal line image, the photomultiplier recorded its intensity profile, i.e. the linespread function (LSF). The scans were relatively slow making this approach unsuited for clinical applications where patient eye movement during a measurement scan will cause errors in the recorded LSF. In our invention, an electronic imaging device such as a vidicon or CCD camera coupled to a computer is used. This configuration results in the acquisition of a retinal image in 1/30 second which is the time required to capture and display a single video frame. The rapid image acquisition time, provided for in our invention, virtually eliminates problems due to eye movement.